Why do some coins have grooves on the edges?
The dollar, half dollar, quarter, and dime were originally made of precious metals (gold and silver). Reeded edges were a way to deter counterfeiting and the practice of filing the edges of coins so the metal shavings could be sold.
The penny and nickel are considered "minor" coins of the United States, so their edges are plain.
Currently, none of the coins produced for circulation contain precious metals, but reeding is useful in another way: reeded edges help people who have visual impairments. For example, the dime and the penny are similar in size, but the reeding on the dime makes it easy to identify by touch.